The present invention relates to a magnetic recording medium exhibiting high output and a good C/N ratio in high-density recording.
In recent years, recording wavelengths have tended to shorten as recording densities have increased. However, the problem of so-called self-magnetization loss where output drops when recording with a short wavelength on a thick magnetic layer has become significant. Thus, the magnetic layer has been reduced in thickness. However, in particulate magnetic recording media, when a magnetic layer equal to or less than 2 xcexcm in thickness is directly coated onto a support, the nonmagnetic support tends to affect the surface of the magnetic layer. As a result, deterioration of electromagnetic characteristics and dropout tend to appear. This problem is solved by a method employing a simultaneous multilayer coating method in which a nonmagnetic layer is provided as a lower layer and magnetic coating liquid with high concentration is thinly applied (for example, see Japanese Unexamined Patent Publication (KOKAI) Showa Nos. 63-191315 and 63-187418). Such inventions have made it possible to achieve particulate magnetic recording media having good electromagnetic characteristics with dramatically improved yields.
In recent popular digital VCR systems and the like, the need to further reduce medium noise in particulate magnetic recording media has emerged. In popular digital VCR systems, surface roughness has been found to affect medium noise at a wavelength pitch of about 4 xcexcm. Further, high output and suitability to overwriting (O/W) is being demanded of the tapes employed in popular digital VCR systems, resulting in further reduction in thickness of the magnetic layer to about 0.1 xcexcm. When the thickness of the magnetic layer is reduced in this manner, for example, the effects on the surface properties of the magnetic layer of factors resulting from the magnetic layer such as aggregation of the magnetic material and orientational turbulence are reduced, with the surface properties of the lower nonmagnetic layer determining the surface properties of the magnetic layer.
Known methods of smoothing the surface of the lower nonmagnetic layer include, for example, the method of employing acicular particles in the lower layer nonmagnetic powder described in Japanese Unexamined Patent Publication (KOKAI) Heisei No. 4-325915, and the method of employing scale-like particles or plate-shaped particles in the lower layer nonmagnetic powder described in Japanese Patent No. 2,698,770. However, in these methods, it is difficult to reduce roughness at a wavelength pitch of from several xcexcm to several tens of xcexcm. Further, when the thickness of magnetic recording tapes is reduced, that is, when the tape length is increased, it is possible to increase the volume density and achieve high density. However, it has been necessary to reduce the thickness of the support relative to metal evaporation (ME) tapes in magnetic recording tapes having a lower nonmagnetic layer such as the present inventions. As the support has been thinned, high-strength supports extended in the direction of width (TD) have come to be employed to achieve good head contact and ensure running durability. However, when such supports develop minute scratches, they tend to sever during coating, calendering, and slitting steps, so that the yield deteriorates, and there are problems with production properties.
From these perspectives, in magnetic recording tapes having a lower nonmagnetic layer, the demand for further thinning of the lower layer is increasing. However, when the lower layer is thinned in the layer configurations described in Japanese Unexamined Patent Publication (KOKAI) Heisei No. 4-325915 and Japanese Patent No. 1698770, calendering molding properties decrease, the effects of protrusions appear on the support surface, aggregates form in the upper and lower layers, additives such as abrasives are exposed, the surface of the magnetic layer is roughened, output drops, and noise increases. In particular, when the lower layer is thinned to 0.1 to 0.5 xcexcm, these effects become marked.
Accordingly, the object of the present invention is to provide a magnetic recording medium exhibiting a high C/N ratio (low noise) in high-density magnetic recording.
The present inventors conducted extensive research into achieving the above-stated object in a magnetic recording medium which comprises a nonmagnetic lower layer comprising a nonmagnetic powder and a binder and a magnetic layer comprising a ferromagnetic powder and a binder provided in this order on a flexible nonmagnetic support, resulting in the discovery that a magnetic recording medium exhibiting a high C/N ratio (lower noise) was obtained by incorporating flat acicular xcex1-iron oxide powder and granular nonmagnetic particles into the nonmagnetic lower layer wherein said flat acicular xcex1-iron oxide powder has a major axis length ranging from 0.05 to 0.5 xcexcm and a ratio of the major width m to the minor width k of the minor axis cross-section when sectioned at an angle perpendicular to the major axis (m/k) is higher than 1, and said granular nonmagnetic particles have a mean particle diameter equal to or less than 0.04 xcexcm; the present invention was devised on that basis.
In the magnetic recording medium of the present invention, the use of flat acicular xcex1-iron oxide powder such as set forth above permits inhibition of surface roughness due to vortex flows by increasing the thixotropic properties of the liquid to a greater level than in conventional acicular xcex1-iron oxide powder. Further, the orientation of the nonmagnetic powder can be increased, permitting the production of a smooth surface during coating film formation. However, the increase in orientation of the nonmagnetic powder decreases voids in the coating film, thereby decreasing calender forming properties, resulting in the problem that the surface property cannot be rendered smooth by calendering.
In the present invention, the combined use of the granular nonmagnetic particles, as described above, ensures the presence of voids in the coating film while inhibiting orientation turbulence of the flat acicular xcex1-iron oxide powder. Therefore, a magnetic recording medium is obtained with a smooth magnetic layer surface property during coating film formation and with improved calendering forming properties.
Further, from the perspective of ensuring voids and increasing the thixotropic property of the liquid, the above-described granular nonmagnetic particles are preferably particles that easily form structures. From this perspective, the granular nonmagnetic particles are most preferably carbon black. However, so long as the particles easily form a structure, they may be ceramic oxides such as titanium dioxide or zirconia oxide, or carbon black may be employed in combination with ceramic oxides. Further, from the perspective of ensuring voids and increasing the thixotropic property of the liquid, the above-mentioned flat acicular particles are preferably particles capable of being dispersed in a small quantity of binder, that is, particles having a low specific surface area in spite of small particle size. From this perspective, the flat acicular particles are preferably xcex1-iron oxide rather than iron oxyhydroxide.
One characteristic of the magnetic recording medium of the present invention is that the nonmagnetic lower layer comprises flat acicular xcex1-iron oxide powder. In the present invention, the flat acicular xcex1-iron oxide powder consists of particles with a major axis length ranging from 0.05 to 0.5 xcexcm, and having a ratio of major width to minor width (major width/minor width) of the minor axis cross-section when sectioned at an angle perpendicular to the major axis of greater than 1. When the major axis length of the flat acicular xcex1-iron oxide powder is less than 0.05 xcexcm, the improvement in surface smoothness decreases. Further, flat acicular xcex1-iron oxide powder with a major axis length exceeding 0.5 xcexcm is difficult to manufacture. The preferred range of the major axis length of the flat acicular xcex1-iron oxide is from 0.07 to 0.3 xcexcm. The shape of the minor axis cross-section when sectioned at an angle perpendicular to the major axis of the flat acicular xcex1-iron oxide powder may be elliptic or polyhedral.
Such a flat acicular xcex1-iron oxide powder tends to be formed so that the lengthwise direction thereof is parallel to the support surface during coating of the nonmagnetic lower layer. Further, incorporating the flat acicular xcex1-iron oxide powder into the lower nonmagnetic layer controls orientation turbulence in the nonmagnetic powder in the direction of thickness (direction perpendicular to the tape web) of the nonmagnetic lower layer better than when a metal nonmagnetic powder with a round minor axis cross-section or a scale-like or plate-shaped nonmagnetic powder is employed. That is, the aggregates caused by entangling of nonmagnetic powder particles in conventional acicular nonmagnetic powder (of round cross-section) can be reduced, and the orientation turbulence of nonmagnetic powder in the direction of thickness (direction perpendicular to the tape web) can be improved relative to what it is in scale-like and plate-shaped nonmagnetic powders of short major axis length.
Thus, the surface roughness during coating film formation is reduced, moldability of the nonmagnetic lower layer during calendering is improved, and it becomes possible to smoothen the surface of the magnetic layer.
Japanese Unexamined Patent Publication (KOKAI) Heisei No. 10-340447 describes a magnetic recording medium having a lower nonmagnetic layer comprising flat acicular particles that are of the same type of powder as the flat acicular nonmagnetic powder employed in the present invention. However, this publication does not describe the mixing of flat acicular xcex1-iron oxide powder with particles that easily form structures, such as carbon black, for use. Further, although the above-cited publication describes iron oxyhydroxide (FeOOH) as a flat acicular particle, xcex1-iron oxide is not disclosed as an flat acicular nonmagnetic powder. Additionally, in the above-cited publication, the thickness of the lower nonmagnetic layer is not specified.
As described in Japanese Unexamined Patent Publication (KOKAI) Heisei No. 4-325915 and Japanese Unexamined Patent Publication (KOKAI) Heisei No. 6-236542, the method of employing carbon black in addition to acicular nonmagnetic particles in the lower layer is actually employed from the perspective of ensuring electrostatic properties and the thixotropic properties of the liquid. When a large quantity of carbon black is added to the lower layer liquid to increase the thixotropic property of the liquid, it sometimes becomes difficult to ensure dispersion. Further, as described in Japanese Unexamined Patent Publication (KOKAI) Heisei No. 10-340447, there is a problem with electrostatic charging when carbon black is not added, sometimes resulting in wrinkling and the like during calendering. Further, the thixotropic property of the liquid decreases, the effects of vortex flow tend to appear during coating and drying, and pitch waviness occurs at long wavelengths (equal to or higher than 10 xcexcm), resulting in surface roughness.
The use of flat acicular nonmagnetic powder improves thixotropic properties to a greater degree than the use of conventional acicular nonmagnetic particles. The effect is particularly marked in systems where carbon black is employed in combination. Further, even when the quantity of carbon black is reduced, it is still possible to ensure the thixotropic property of the liquid, permitting improved dispersion. Further, the combined use of carbon black and a ceramic oxide as the granular nonmagnetic particles achieves both dispersibility and thixotropic properties.
Since heat treatments are not conducted at high temperatures for iron oxyhydroxide, there is greater specific surface area and lower compression strength for the same particle size as compared to xcex1-iron oxide. When the specific surface area increases in this manner, the quantity of binder required to ensure dispersibility of the liquid becomes large. When the quantity of binder in the lower layer becomes 30 parts by weight per 100 parts by weight of nonmagnetic powder as in the embodiment described in Japanese Unexamined Patent Publication (KOKAI) Heisei No. 10-340447, it becomes impossible to ensure adequate calendering moldability. Further, when iron oxyhydroxide is employed, there are cases where the acicular particles break during the application of high pressure during the calendering process.
Accordingly, the present inventors sintered and hydrated the flat acicular iron hydroxide described in Japanese Unexamined Patent Publication (KOKAI) Heisei No. 10-340447 and Japanese Unexamined Patent Publication (KOKAI) Heisei No. 10-340805 at 600xc2x0 C. in an O2 atmosphere, for example, as flat nonmagnetic powder to manufacture flat acicular xcex1-iron oxide.
In the present invention, not only flat acicular xcex1-iron oxide is employed as the lower layer nonmagnetic powder, but also a granular particle such as carbon black is mixed in to ensure electrostatic properties and permit the production of a magnetic recording medium with little surface roughness during coating film formation.
In-the magnetic recording medium of the present invention, the following conditions are preferably satisfied to further enhance the above-described effects of the present invention.
A ratio of the minor width length k of the minor axis cross-section when flat acicular xcex1-iron oxide is sectioned in a direction perpendicular to the major axis thereof to the major axis length 1x of the powder (1x/k) equal to or higher than 5, preferably equal to or higher than 7, and more preferably equal to or higher than 10 is desirable from the perspective of enhancing the thixotropic property of the liquid and reducing displacement in the direction of thickness due to particle stacking.
A ratio of the major width length m of the minor axis cross-section when the flat acicular xcex1-iron oxide is sectioned in a direction perpendicular to the minor width length k (major width length m/minor width length k) exceeding 1, more preferably from 1.5 to 8, and still more preferably from 2 to 5 is desirable from the perspective of facilitating orientation in parallel with the support surface and imparting an ability to receive pressure during calendering.
A ratio of the major axis length 1x of the flat acicular xcex1-iron oxide powder to the mean particle diameter 1y of the granular nonmagnetic powder (1x/1y) equal to or higher than 3, preferably equal to or higher than 5, and more preferably equal to or higher than 10 is desirable from the perspective of inhibiting orientation turbulence due to particle stacking during mixing.
A ratio of the major width length m of the minor axis cross-section when the flat acicular xcex1-iron oxide powder is sectioned in a direction perpendicular to the major axis to the mean particle diameter 1y of the granular nonmagnetic powder (m/1y) of from 0.5 to 10, preferably from 1 to 8, and still more preferably from 2 to 6, is desirable. Thus, it is possible to reduce displacement in the direction of thickness due to particle stacking during mixing, smoothen the surface, and achieve a suitable surface resistivity (Rs) value.
A ratio of the minor width length k of the minor axis cross-section of the flat acicular xcex1-iron oxide to the mean particle diameter 1y of the granular nonmagnetic powder (k/1y) of from 0.3 to 2, preferably from 0.5 to 1.5, and still more preferably from 0.8 to 1.2, is desirable from the perspective of ability to inhibit the stacking of granular particles and flat acicular xcex1-iron oxide powder.
A ratio of the lower layer thickness d to the major axis length 1x of the flat acicular xcex1-iron oxide powder (d/1x) of from 0.05 to 25, preferably from 0.1 to 10, and still more preferably from 0.1 to 4 is desirable to improve orientation properties in the direction of thickness and render the surface smooth. When d/1x drops below 4, when present in the thickness direction, granular particles hardly be inserted between the particles of flat acicular xcex1-iron oxide powder and both the flat acicular (xcex1-iron oxide particles and granular particles such as carbon black are present on the same flat surface. That is, the flat acicular xcex1-iron oxide particles are randomly oriented within the surface and granular particles are present in the gaps therebetween. Thus, not only is the surface rendered smooth, but the strength of the lower layer in the width direction is increased, and it is possible to effectively flatten the upper magnetic layer during calendering, and running durability such as resistance to edge damage with repeat running when applied as a tape can be improved.
From such perspectives, inorganic compounds other than carbon black, such as titanium oxide, may be employed as the granular nonmagnetic particles. However, in this case as well, the particle diameter of the granular nonmagnetic particles must be equal to or less than 0.04 xcexcm as specified in the present invention.
The content of flat acicular xcex1-iron oxide powder preferably ranges from 10 to 95 parts, more preferably from 60 to 95 parts per 100 parts by weight of the total quantity of nonmagnetic powder. When the major axis length of the flat acicular xcex1-iron oxide particles is made 1, the average thickness d of the nonmagnetic lower layer becomes thinner, the ratio d/1 drops, there is improvement with regard to entanglement of nonmagnetic particles, and variation in the orientation of the particles in the direction of thickness decreases.
The average thickness of the upper magnetic layer preferably ranges from 0.01 to 0.5 xcexcm, more preferably from 0.04 to 0.3 xcexcm, and most preferably from 0.04 to 0.15 xcexcm. When the upper layer thickness exceeds 0.5 xcexcm, the smoothening effect of the lower layer diminishes and smoothening of the magnetic layer surface tends to decrease.
To increase reduction of medium noise, employing a support in which wavelength roughness components of from 1 to 10 xcexcm in the surface roughness of the side on which the magnetic layer will be provided have been reduced yields an even greater effect. When the ratio of lower layer thickness d to major axis length 1x of the flat acicular xcex1-iron oxide powder (d/1x) is equal to or less than 4, the effect is even greater.
Flat acicular xcex1-iron oxide powder can be manufactured by the method , as described in Japanese Unexamined Patent Publication (KOKAI) Heisei No. 10-340805, in which water-soluble Al salts and salts of other rare earth metals such as Y are added to a reaction system generating iron oxyhydroxide to prepare Co-containing iron oxyhydroxide in which Al, Y, and other rare earth metals have been dissolved as solids. In this process, flat acicular iron oxyhydroxide can be prepared by adjusting the proportion of Co/Al/Y and other rare earth metals. Flat acicular xcex1-iron oxide can be prepared by sintering in O2 gas.
In the Co-containing iron oxyhydroxide in which Al and rare earth metals such as Y are dissolved as solids, it is preferable that the Co content ranges from 5 to 50 atomic percent, preferably from 20 to 35 atomic percent; the Al content ranges from 0.1 to 12 atomic percent, preferably from 3 to 8 atomic percent; the content of rare earth metals such as Y ranges from 0.1 to 12 atomic percent, preferably from 3 to 8 atomic percent; and the atomic ratio of the Al content to the content of rare earth metals such as Y preferably ranges from 0.5 to 2 to effectively increase the ratio of the major width/minor width.
As described in Japanese Unexamined Patent Publication (KOKAI) Heisei No. 10-340447, it is also possible to prepare flat xcex1-iron oxide by that water-soluble Al salts and Si salts are added to prepare the iron oxyhydroxide, and then this iron oxyhydroxide is sintered in O2 gas. However, as described in Japanese Unexamined Patent Publication (KOKAI) Heisei No. 10-340805, the incorporation of Co/Al/Y or some other rare earth metal is preferred because it facilitates microgranulation and flatening.
The quantity of binder resin employed in the lower nonmagnetic layer ranges from 5 to 30 parts by weight per 100 parts by weight of total nonmagnetic powder to ensure moldability in the calendering process and dispersion in the liquid. When the quantity of binder resin becomes excessive, moldability in the calendering process tends to decrease. When the quantity of binder resin becomes excessively low, dispersion tends to decrease. Thus, the above-stated range is preferred.
When applying a thin lower layer, coating can be readily achieved by reducing the concentration of lower layer nonmagnetic liquid. However, when the concentration of the lower layer nonmagnetic liquid is decreased, a problem occurs in the form of aggregation of the nonmagnetic particles in the liquid. Thus, as a binder, it is further preferred to use polyurethane resin having a polar group, polyurethane comprising this polyurethane resin in a cyclic structure as well as an ether group, branched aliphatic polyester, polyurethane, polyurethane having a dimer diol structure, or the like is preferably employed as the binder. Since these binders are adsorbed onto the particle surfaces and form long molecular chains having adequate hardness, the gap between particles in the liquid can be widened and the aggregation property of the particles can be inhibited. Further, because aggregation of particles during coating and drying can be inhibited, a coating film with little aggregation turbulence of particles can be formed. One type or a mixture of these urethane resins may be employed. However, the compositional ratio of the urethane resin in the binder of the nonmagnetic layer is preferably equal to or higher than 10 weight percent, more preferably equal to or higher than 20 weight percent.
From the perspective of increasing granular orientation during coating, the solid component concentration of the liquid is further preferably adjusted to lower the ratio h/l of the magnetic liquid film h to the major axis length l of the flat acicular magnetic particles immediately after coating. Further, smoothing the magnetic layer surface with a smooth material at the stage where the coating has somewhat dried can increase the particle orientation in the upper and lower layers. When increasing the coating speed or employing an extrusion coating method, it is preferable that gither slit shapes are devised and a shearing force is applied to the coating liquid to break up aggregation of particles. Further, granular orientation can be further improved by moderating the initial drying of coating to inhibit vortex flow of the coating liquid.
Calendering is preferably conducted as follows. The initial roll nips are configured of metal rolls and the linear nip pressure is equal to or higher than 300 kg/cm, preferably equal to or higher than 400 kg/cm. The processing speed is equal to or less than 150 m/min, preferably equal to or less than 100 m/min, and still more preferably equal to or less than 30 m/min. The temperature ranges from 70 to 100xc2x0 C. it is preferable to set the above conditions appropriately in consideration for the ease of molding the upper and lower layers affected by the Tg, kind and quantity of binder.